This invention relates generally to standby power systems and more particularly, it relates to an improved standby power supply system which includes a variable frequency inverter so as to reduce stress of the components thereof during its initial start-up.
As is generally known in the design of off-line standby power systems, there has been encountered a problem of how to reduce the stress that exists on the inverter components during the transition from a standby mode of operation to an active backup mode of operation. There are two major sources that cause this stress: (1) the activation of the solid-state inverter before the input line connection is completely disconnected from the utility power source, and (2) the saturation of the magnetic components in the inverter or load. Thus, the standby power system designers must be careful so as to take into consideration this stress factor. Otherwise, the stress could cause a failure of the inverter at a critical point of operation of the standby power system.
The first source for the stress is due to the fact that a fast-acting electro-mechanical relay is usually employed in low-cost standby power systems for transferring the line connection. The relay of this type will typically be able to sustain an arc across its contacts until the applied current crosses through zero. However, since the back-up power system is attempting to power both the main power source (grid) as well as the protected load during this arcing interval, there will be created high peak currents which reduce the inverter output voltage to the load and may also cause damage to the inverter components.
To date, the prior art solutions to the first source of the stress have involved either the use of a solid-state switch for performing the line disconnecting function (transfer operation) or the delaying of the turn-on of the inverter for a period of time which is greater than a half-cycle of the line frequency. Although the use of the solid-state switch eliminates the arcing, it has the disadvantage of a substantial increase in cost. While the delayed turn-on of the inverter ensures that a zero-crossing will occur, this approach suffers from the disadvantage of adding additional delay to the already existing system delays (i.e., line detector response time, relay contact transfer time, etc.) which increases the minimum acceptable utility voltage or requires greater reserve capacity in the load power supply.
The stress created by the second source is the result of high current pulses being generated upon the initial turn-on of the back-up power source which may damage or physically destroy components in the inverter as well as reducing the available voltage to the load. Further, this problem becomes even more aggravated when the inverter waveform is a squarewave or stepped waveform since this does not insure a balance of flux in the magnetic components. In the past, there has been made an attempt to solve this problem by using a sine-wave inverter which is phase-synchronized to the primary power source or grid. While this technique is effective in minimizing the possibility of saturation due to the flux imbalance, there is created the drawbacks of a significant increase in system costs and overall complexity due to the need for the phase locking and sine-wave generation.
Another prior art method for preventing saturation is to provide means for monitoring peak currents and for shutting down the inverter for a period of time equal to the utility line frequency or for decreasing the pulse width upon the detection of excessive current peaks. There is a serious disadvantage in this prior art method since it extends the period when the backup power is not being supplied to the load.
It would therefore be desirable to provide an improved standby power supply system which prevents saturation of the magnetic components thereof during its initial start-up. Further, it would be expedient that the standby power supply system be manufactured with a relatively small increase in the overall system costs.